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Kelly Criterion Position Sizer

Enter your bankroll, the market price, and your estimated probability to see how much to stake.

$
$
$0.01$0.50$0.99
%
1%50%99%
Buy YesEdge: 10.00%

Suggested Stake (Half Kelly)

$100.00

Number of Shares

200.00

Bankroll at Risk

10.00%

Kelly Criterion for binary markets: f* = (p - price) / (1 - price) for Buy Yes, or f* = (price - p) / price for Buy No. This calculator uses half Kelly (0.5×) to reduce variance.

How the Kelly Criterion Works

Kelly answers a simple question: given a favorable bet, how much should you risk? Bet too little and you leave growth on the table; bet too much and a bad streak can break your bankroll.

For a binary market, Kelly simplifies to f* = (p − price) / (1 − price) when buying Yes, where p is your estimated probability and price is the market price. This calculator uses half Kelly (0.5×) to keep variance low while retaining most of the growth.

Frequently Asked Questions

What is the Kelly Criterion?
Kelly is a formula for how much of your bankroll to risk on a favorable bet. It balances growing your money against the risk of ruin, using the market price, your probability estimate, and your bankroll.
Why does this use half Kelly?
Full Kelly maximizes theoretical growth but assumes your probability estimate is exact. Half Kelly keeps most of the growth benefit with far less variance, which is why professionals favor it.
What if I bet more than Kelly suggests?
Betting more than full Kelly lowers your long-run growth; at 2x Kelly it drops to zero and beyond that it turns negative. Stay at or below the suggested stake.